1. Field of the Invention
The present invention relates generally to frequency changer circuits for electric motor systems, and more specifically, the present invention relates to the calculation, generation, application and control of currents to neutralize unwanted harmonic frequencies in conventional frequency changer circuits.
2. Description of the Background
In many diverse environments, electric motors are used as drive or propulsion systems. The requirement for low noise and low distortion power conversion systems has made the multilevel PWM (pulse width modulation) technology a strong contender for many different electric motor drive applications. However, the efficiency and power density of this technology is limited by the need for two to three stages of power conversion. Specifically, power must first be converted from AC to DC and then from DC back to AC by accommodating the PWM technology.
These multistage conversions hinder system efficiency and lower power density. For example, the losses in the conversion processes typically range from 2-3% of applied power. Moreover, the power density for the PWM multilevel inverter has traditionally remained in the range of from 0.5 to just over 1.0 MW/m3. In fact, when all of the equipment required for the total conversion process, (e.g., transformers, rectifiers, inverters and filters) are included in the power density calculation, the system power density is typically in the range of 0.5-0.75 MW/m3.
In many applications, these losses are not acceptable. For example, any wasted power in “onboard” or self-sustained systems such as ships and submarines significantly decreases propulsion system performance. Where power is at a minimum, the present invention finds its most effective applications.
In addition to these specific propulsion applications, improvements in the power density via a reduction in losses is continually sought in all electric motor arts. As such, the present invention preferably provides a system architecture capable of improving the power density of electric motors over traditional PWM systems by a factor of at least 3-6 times and reducing system losses to about 1%.